The invention relates to inductor arrangements comprising an input inductor coupled to the input side of an electrical apparatus and an output inductor coupled to the output side of the apparatus.
Input and output inductors are used to reduce interference that an electrical apparatus causes to the input and output side networks. Input and output inductors are used for instance in frequency converter configurations.
In frequency converters, rectification is typically implemented by means of a six-pulse diode bridge, which is known to use line current only at the surroundings of the peak voltage of a sequence, thus causing extensive current pulses that stress the network. In order to reduce the amplitude of these current pulses it is known in the art to use series inductors, i.e. input inductors, placed in the feeding phases.
Power inversion and pulse-width modulation used to control the output voltage level of the fundamental wave cause extremely rapidly ascending and descending edges, a kind of surge waves, to the output voltage. These surge waves may create two types of problems in the motor to be fed: high turn voltages of the winding including the risk of discharge and bearing currents. In order to attenuate each of the mentioned phenomena it is known in the art to employ phase-specific series inductors, i.e. output inductors, to be placed at the beginning of a motor cable at the output side of the frequency converter that allow smoothing the voltage edges observable in the terminals of the motor.
An input inductor is generally a three-columned and two-windowed three-phase inductor assembled of columns and yokes composed of armature sheets and copper or aluminium windings. The magnetic path is provided with one or more air gaps that prevent the magnetic core from being saturated. Such a component intended for a network frequency is typically the largest and heaviest part of the entire converter.
The output inductor that smoothes the surge waves observable in the terminals of the motor could electrically be most optimal when it would only affect with frequency components of such a magnitude that only the edges of the surge voltages were smoothed.
The structure of an output inductor according to the prior art is similar to the input inductor. However, such an output inductor also attenuates a component of base frequency, whereby the terminal voltage of the motor is reduced. Such an inductor is also so massive that it cannot be placed into the specific frequency converter as an optional component, instead it is separately mounted.
Output inductor structures are also known which are effective only in high frequency components. What are used are for instance rings made of a material provided with an extremely high specific permeability that positioned around output busbars attenuate the voltage transients. A drawback with these components is that they are very expensive. Consequently they are generally used only as a “common mode” inductor, which is common for all phases, whereby the effect is restricted merely to prevent bearing currents. Another problem with such rings is the relatively large size thereof.
Another structure in use, which is only effective in large frequency components, comprises an inductor bar provided with an open magnetic path placed in each output phase, the structure of such an inductor bar resembles a winding around a pile of armature sheets. The problems associated with this structure include high costs and a fairly extensive need for space.